Catalyst for curing epoxides

ABSTRACT

The use of 1,3-substituted imidazolium salts of the formula I 
     
       
         
         
             
             
         
       
     
     in which
 
R1 and R3 independently of one another are an organic radical having 1 to 20 C atoms,
 
R2, R4, and R5 independently of one another are an H atom or an organic radical having 1 to 20 C atoms, it also being possible for R4 and R5 together to form an aliphatic or aromatic ring, and
 
X is a thiocyanate anion
 
as latent catalysts for curing compositions comprising epoxy compounds.

The invention relates to the use of 1,3-substituted imidazolium salts ofthe formula I

in whichR1 and R3 independently of one another are an organic radical having 1to 20 C atoms,R2, R4, and R5 independently of one another are an H atom or an organicradical having 1 to 20 C atoms, it also being possible for R4 and R5together to form an aliphatic or aromatic ring, andX is a thiocyanate anionas latent catalysts for curing compositions comprising epoxy compounds.

Epoxy compounds are used for producing coatings, as adhesives, forproducing shaped articles, and for numerous other purposes. In theseapplications they are generally present during processing in a liquidform (as solutions in suitable solvents or as liquid, solvent-free 100%systems). The epoxy compounds are generally of low molecular mass. Inthe context of the use they are cured. Various possibilities are knownfor curing. Starting from epoxy compounds having at least two epoxygroups it is possible, with an amino compound or with an acid anhydridecompound having at least two amino groups or at least one anhydridegroup, respectively, for curing to take place through a polyadditionreaction (chain extension). Amino compounds or acid anhydride compoundsof high reactivity are generally not added until shortly before thedesired curing. The systems in question are therefore referred to astwo-component (2K) systems.

Additionally it is possible to use catalysts for the homopolymerizationor copolymerization of the epoxy compounds. Known catalysts includecatalysts which are only active at high temperatures (latent catalysts).Latent catalysts of this kind have the advantage of allowingone-component (1K) systems; in other words, the epoxy compounds cancomprise the latent catalysts without any unwanted premature curingoccurring.

Latent catalysts available commercially include, in particular, adductsof boron trifluoride with amines (BF₃-monoethylamine), quaternaryphosphonium compounds, and dicyandiamide (DICY).

Journal of Polymer Science: Polymer Letters Edition, vol. 21, 633-638(1983) describes the use of 1,3-dialkylimidazolium salts for thispurpose. Their decomposition above 175° C. releases 1-alkylimidazoles,which then bring about the cure. The structure of the cation has beenvaried, and the halides chloride and iodide have been used as anions.

DE-A 2416408 discloses imidazolium borates, such as imidazoliumtetraphenylborate or imidazolium tetra-n-butylborate.

U.S. Pat. No. 3,635,894 describes 1,3-dialkylimidazolium salts withanions selected from chlorides, bromides, and iodides as latentcatalysts for epoxy compounds.

Kowalczyk and Spychaj, Polimery (Warsaw, Poland) (2003), 48(11/12),833-835 describe the use of 1-butyl-3-methylimidazoliumtetrafluoroborate as a latent catalyst for epoxy compounds. The activityof the catalyst does not begin until 190° C.

Sun, Zhang and Wong, Journal of Adhesion Science and Technology (2004),18(1), 109-121 disclose the use of 1-ethyl-3-methylimidazoliumhexafluorophosphate as a latent catalyst. The activity begins only at196° C.

JP 2004217859 uses imidazolium tetraalkylborates or imidazoliumdialkyldithiocarbamates. The activation takes place by exposure tohigh-energy light.

EP 0 458 502 discloses a multiplicity of very different catalysts forepoxy compounds. Included in the list are1-ethyl-2,3-dimethylimidazolium acetate (R1=ethyl, R2=methyl, andR3=methyl in formula I) and 1-ethyl-2,3-dimethylimidazoliumacetate-acetic acid complex.

Suitable latent catalysts ought to be readily miscible with the epoxycompounds. The mixtures ought to remain stable for as long as possibleat room temperature under standard storage conditions, so that they aresuitable as storable 1K systems. In the context of the use, however, thetemperatures required for curing should not be excessively high, and inparticular should be well below around 200° C. Lower curing temperaturesallow energy costs to be saved and unwanted secondary reactions avoided.In spite of the lower curing temperature, the mechanical and performanceproperties of the cured systems ought as far as possible not to beimpaired. The desire is that these properties (examples being hardness,flexibility, bond strength, etc) should be at least as good and ifpossible even better.

An object of the present invention, therefore, were imidazolium salts aslatent catalysts, and mixtures of these imidazolium salts and epoxycompounds meeting the requirements set out above.

Found accordingly have been the above-defined use of the latentcatalysts of the formula I, and compositions comprising the latentcatalysts.

The Imidazolium Salts

The invention uses 1,3-substituted imidazolium salts of the formula I

in whichR1 and R3 independently of one another are an organic radical having 1to 20 C atoms,R2, R4, and R5 independently of one another are an H atom or an organicradical having 1 to 20 C atoms, in particular 1 to 10 C atoms, it alsobeing possible for R4 and R5 together to form an aliphatic or aromaticring, andX is a thiocyanate anion.

R1 and R3 independently of one another are preferably an organic radicalhaving 1 to 10 C atoms. The organic radical may also comprise furtherheteroatoms, more particularly oxygen atoms, preferably hydroxyl groups,ether groups, ester groups or carbonyl groups.

In particular R1 and R3 independently of one another are a hydrocarbonradical which apart from carbon and hydrogen may at most furthercomprise hydroxyl groups, ether groups, ester groups or carbonyl groups.

R1 and R3 are preferably independently of one another a hydrocarbonradical having 1 to 20 C atoms, more particularly having 1 to 10 Catoms, which comprises no other heteroatoms, oxygen or nitrogen forexample. The hydrocarbon radical may be aliphatic (in which caseunsaturated aliphatic groups are also included) or aromatic, or maycomprise both aromatic and aliphatic groups.

Examples of hydrocarbon radicals that may be mentioned include phenylgroup, benzyl group, a phenyl or benzyl group substituted by one or moreC1 to C4 alkyl groups, or alkyl groups and alkenyl groups, moreparticularly the allyl group.

With particular preference R1 and R3 independently of one another are aC1 to C10 alkyl group, an allyl group or a benzyl group. A particularlypreferred alkyl group is a C1 to C6 alkyl group, and in one particularembodiment the alkyl group is a C1 to C4 alkyl group.

With very particular preference R1 and R3 independently of one anotherare a methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl ortert-butyl group, an allyl group or a benzyl group, particularimportance attaching to the methyl, ethyl, n-propyl, and n-butyl group.

In one particular embodiment

R1 and R3 are a methyl group,R1 and R3 are an ethyl group,R1 is a methyl group and R3 is an ethyl group,R1 is a methyl group and R3 is an n-propyl group,R1 is a methyl group and R3 is an n-butyl group,R1 is a methyl group and R3 is an allyl group,R1 is an ethyl group and R3 is an allyl group,R1 is a methyl group and R3 is a benzyl group,R1 is an ethyl group and R3 is a benzyl group.

R2, R4, and R5 independently of one another are an H atom or an organicradical having 1 to 20 C atoms, it also being possible for R4 and R5together to form an aliphatic or aromatic ring. Besides carbon andhydrogen, the organic radical may also comprise heteroatoms such asnitrogen or oxygen; preferably it can comprise oxygen, more particularlyin the form of hydroxyl groups, ester groups, ether groups or carbonylgroups.

More particularly, R2, R4, and R5 independently of one another are an Hatom or a hydrocarbon radical which apart from carbon and hydrogen mayat most further comprise hydroxyl groups, ether groups, ester groups orcarbonyl groups.

R2, R4, and R5 are preferably independently of one another a hydrogenatom or a hydrocarbon radical having 1 to 20 C atoms, more particularlyhaving 1 to 10 C atoms, which comprises no other heteroatoms, oxygen ornitrogen for example. The hydrocarbon radical may be aliphatic (in whichcase unsaturated aliphatic groups are also included) or aromatic, or maybe composed of both aromatic and aliphatic groups, in which case R4 andR5 may also form an aromatic or aliphatic hydrocarbon ring, which ifappropriate may be substituted by further aromatic or aliphatichydrocarbon groups (the number of C atoms of the unsubstituted orsubstituted hydrocarbon ring, including the substituents, may in thiscase preferably be not more than 40, in particular not more than 20,with particular preference not more than 15, or not more than 10).

Examples of hydrocarbon radicals that may be mentioned include thephenyl group, a benzyl group, a phenyl or benzyl group substituted byone or more C1 to C4 alkyl groups, or alkyl groups, alkenyl groups, and,if R4 and R5 form a ring, an aromatic 5- or 6-membered ring formed by R4and R5, or a cyclohexene or cyclopentene, it being possible for thesering systems in particular to be substituted by one or more C1 to C10,more particularly C1 to C4, alkyl groups.

With particular preference R2, R4, and R5 independently of one anotherare an H atom, a C1 to C8 alkyl group, a C1-C8 alkenyl group, such as anallyl group, or a phenyl group or a benzyl group.

With very particular preference R2, R4, and R5 independently of oneanother are an H atom or a methyl, ethyl, n-propyl, isopropyl, n-butyl,sec-butyl or tert-butyl group, particular importance attaching to themethyl, ethyl, n-propyl, and n-butyl group.

In one particular embodiment R2, independently of the other radicals R4and R5 and the remaining radicals R1 and R3, is an H atom. Imidazoliumsalts of the formula I in which R2 is an H atom are particularlyadvantageous in the context of the present invention: they have goodsolubility in the epoxy compounds and a high activity as latentcatalyst.

In one particular embodiment

R2, R4, and R5 are an H atom,R2 is an H atom or a C1 to C4 alkyl group and R4 and R5 are each an Hatom or a C1 to C4 alkyl group.

Specific cases of the cations of the compounds of the formula I that maybe mentioned include the following:

-   1-butyl-3-methylimidazolium (R1=butyl, R3=methyl)-   1-butyl-3-ethylimidazolium (R1=butyl, R3=ethyl)-   1,3-dimethylimidazolium (R1=methyl, R3=methyl)-   1-ethyl-3-methylimidazolium (R1=ethyl, R3=methyl)-   1-ethyl-2,3-dimethylimidazolium (R1=ethyl, R2=methyl, R3=methyl)

X is a thiocyanate anion. Thiocyanate anions are also known under thename rhodanides. They can be present in the mesomeric forms (S—C≡N)⁻ or(S═C═N)⁻. The term thiocyanate is understood as meaning each of themesomeric forms and also mixtures thereof.

Imidazolium salts of the formula I are available commercially—forexample, from the companies BASF, Sigma Aldrich or Merck. The anions ofthe salts available can easily be replaced by other anions by means ofion exchange, if desired.

The Epoxy Compounds

The curable composition comprises epoxy compounds. Particularly suitableepoxy compounds are those having 1 to 10 epoxy groups, preferably havingat least 2 epoxy groups.

With particular preference the curable composition comprises epoxycompounds having 2 to 6, very preferably having 2 to 4, and inparticular having 2 epoxy groups.

The epoxy groups are, in particular, glycidyl ether groups of the kindformed in the reaction of alcohol groups with epichlorohydrin.

The epoxy compounds may be low molecular mass compounds, which ingeneral have an average molar weight Mn of less than 1000 g/mol, orcompounds of higher molecular mass (polymers). They may be aliphaticcompounds, including cycloaliphatic compounds, or compounds containingaromatic groups.

In particular the epoxy compounds are compounds having two aromatic oraliphatic 6-membered rings, or oligomers thereof.

Of significance in the art are epoxy compounds which are obtainable byreacting epichlorohydrin with compounds which have at least two reactiveH atoms, more particularly with polyols.

Of particular significance in the art are epoxy compounds which areobtainable by reacting epichlorohydrin with compounds which comprise atleast two, preferably two, hydroxyl groups and two aromatic or aliphatic6-membered rings; such compounds include, in particular, bisphenol A andbisphenol F, and also hydrogenated bisphenol A and bisphenol F.

Also suitable are reaction products of epichlorohydrin with otherphenols, such as with cresols or phenol-aldehyde adducts, such asphenol-formaldehyde resins, more particularly novolaks.

Also suitable, of course, are epoxy compounds which derive their epoxycompounds not from epichlorohydrin. Suitable examples include epoxycompounds which comprise epoxy groups through reaction with glycidyl(meth)acrylate, e.g., free-radical copolymerization with glycidyl(meth)acrylate. Mention may also be made in this context of ERL-4221from Dow (CAS Number 2386-87-0):

Epoxy compounds that are suitable for the use of the compositions aremore particularly those which are liquid at processing temperatures of20 to 100° C., more preferably at 20 to 40° C., very preferably at 20°C.

Other Ingredients of the Compositions

The composition of the invention may comprise other ingredients as wellas the latent catalyst and the epoxy compound.

The composition is suitable for 1 K systems or else as a storablecomponent for 2 K systems.

In the case of 2 K systems a second, highly reactive component is onlyadded shortly before use; following the addition of the 2nd components,the resulting mixture is no longer storage-stable, because thecrosslinking reaction or curing begins and leads to an increase inviscosity.

1 K systems already comprise all of the necessary ingredients, and arestable on storage.

The remarks below relating to the composition apply both to 1 K and 2 Ksystems, unless specifically stated otherwise.

As well as the epoxy compounds, the composition may comprise furtherreactive or nonreactive ingredients.

Suitable examples include phenolic resins; phenolic resins here arecondensation products of phenol or derivatives of phenol, e.g., o-, m-or p-cresol, and aldehydes or ketones, more particularly formaldehyde.Particularly suitable phenolic resins are resoles and more particularlywhat are called novolaks, which are phenolic resins obtainable by acidiccondensation of phenol or cresols with formaldehyde, more particularlywith a molar excess of the phenol. The novolaks are preferably solublein alcohols or acetone.

Also suitable are anhydride crosslinkers such as phthalic anhydride,trimellitic anhydride, benzophenonetetracarboxylic dianhydride,tetrahydrophthalic anhydride, hexahydrophthalic anhydride,4-methyltetrahydrophthalic anhydride, 3-methyltetrahydrophthalicanhydride, 4-methylhexahydrophthalic anhydride or3-methylhexahydrophthalic anhydride.

The phenolic resins and anhydride curing agents crosslink with epoxycompounds in the form of a polyaddition. This polyaddition reaction aswell, more particularly the polyaddition reaction of the epoxy compoundswith the phenolic resin, is accelerated by the imidazolium salt of theformula I.

Particularly suitable compositions of the invention hence also includethose which as well as the imidazolium salt of the formula I and theepoxy compound also comprise at least one phenolic resin, preferably anovolak.

Nonreactive ingredients include resins which do not enter into anyfurther crosslinking reaction, and also inorganic fillers or pigments.

The composition may also comprise solvents. Suitability is possessed by,if appropriate, organic solvents, in order to set desired viscosities.

In one preferred embodiment the composition comprises solvents, if atall, in minor amounts (less than 20 parts by weight, more particularlyless than 10 or less than 5 parts by weight per 100 parts by weight ofepoxy compound), and with particular preference does not comprisesolvent (100% system).

Preferred compositions are composed of at least 30% by weight,preferably at least 50% by weight, very preferably at least 70% byweight, of epoxy compounds (in addition to any solvents used).

The amount of the imidazolium salt of formula I is preferably 0.01 to 10parts by weight per 100 parts by weight of epoxy compound, morepreferably at least 0.1, in particular at least 0.5, and very preferablyat least 1 part by weight per 100 parts by weight of epoxy compound; theamount is preferably not higher than 8 parts, in particular not higherthan 6 parts by weight per 100 parts by weight of epoxy compound, and inparticular the amount may also, for example, be 1 to 6 or 3 to 5 partsby weight per 100 parts by weight of epoxy compound.

As well as the imidazolium salts of the formula I, the composition mayof course also comprise further latent catalysts already known hitherto,examples being adducts of boron trifluoride with amines(BF3-monoethylamine), quaternary phosphonium compounds or dicyandiamide(DICY).

The composition is preferably liquid at processing temperatures of 20 to100° C., more preferably at 20 to 40° C., very preferably at 20° C.

The increase in viscosity of the overall composition at temperatures upto 50° C. over a period of 10 hours, in particular of 100 hours (fromthe addition of the latent catalyst), is less than 20%, more preferablyless than 10%, very preferably less than 5%, more particularly less than2%, based on the viscosity of the composition without the latentcatalyst at 21° C. and 1 bar.

The above composition is suitable as a 1 K system.

It is also suitable as a storable component of a 2 K system.

In the case of the 2 K systems only highly reactive components, such asconventional, highly reactive amine curing agents or reactive anhydridecuring agents, are added prior to use; thereafter, curing begins and isevident from an increase in viscosity.

Suitable examples include reactive polyamines or polyanhydrides whichare typically used as crosslinkers for epoxy compounds in 2 K systems.Known amine crosslinkers are, in particular, aliphatic polyamines suchas diethylenetriamine, triethylenetetraamine or amines based onpropylene oxide and ammonia (polyetheramines).

Curing and Use

The compositions which comprise imidazolium salts of the formula I arestable on storage. The imidazolium salts of the formula I are readilysoluble in the epoxy compounds and in the compositions of the invention.The imidazolium salts of the formula I are active in the compositions aslatent catalysts. Their efficiency in the polymerization or crosslinkingof the epoxy compounds is very good.

At typical storage temperatures below 40° C., more particularly below30° C., there is very little or no increase observed in the viscosity ofthe compositions. The compositions are therefore suitable as 1 Ksystems. 1 K systems, prior to their use, do not require the addition ofa 2nd component to bring about curing or crosslinking.

The compositions are of course also suitable as a storable component for2 K systems (see above).

The curing of the compositions, as a 1 K system or as a 2 K system, cantake place at temperatures lower than has been possible with the latentimidazolium catalysts known to date. Curing can take place atatmospheric pressure and at temperatures less than 250° C., inparticular at temperatures less than 200° C., preferably at temperaturesless than 175° C., more preferably at temperatures less than 150° C.,and very preferably at temperatures less than 125° C., and even lessthan 100° C. Curing at temperatures less than 80° C. is also possible.Curing may take place in particular in a temperature range from 40 to175° C., more particularly from 60 to 150° C., or from 60 to 125° C.

The compositions of the invention are suitable for use as a coating orimpregnating composition, as an adhesive, composite material, forproducing shaped articles, or as casting compounds for embedding,attaching or solidifying shaped articles. This and the remarks belowapply both to the 1 K systems and to 2 K systems; preferred systems forall of the stated applications are the 1 K systems.

Examples of coating compositions include paints and varnishes. Using thecompositions of the invention (1 K or 2 K) it is possible in particularto obtain scratch-resistant protective coatings on any desiredsubstrates, made from metal, plastic or wood-based materials, forexample. The compositions are also suitable as insulating coatings inelectronic applications, such as insulating coatings for wires andcables, for example. Mention may also be made of their use for producingphotoresists. They are also suitable, in particular, as refinish coatingmaterial, including in connection, for example, with the renovation ofpipes without their disassembly (curing in place pipe (CIPP)rehabilitation). They are additionally suitable for the sealing offloors.

Adhesives include 1 K or 2 K structural adhesives. Structural adhesivesserve to connect shaped parts permanently to one another. The shapedparts may be of any desired material: suitable materials includeplastic, metal, wood, leather, ceramic, etc. The adhesives in questionmay also be hot melt adhesives, which are fluid and can be processedonly at a relatively high temperature. They may also be flooringadhesives. The compositions are also suitable as adhesives for producingprinted circuit boards (electronic circuits), not least by the SMTmethod (surface-mounted technology).

In composites, different materials, such as plastics and reinforcingmaterials (fibers, carbon fibers), are joined to one another.

The compositions are suitable, for example, for producing preimpregnatedfibers, e.g., prepregs, and for their further processing to composites.

Production methods for composites include the curing of preimpregnatedfibers or woven fiber fabrics (e.g., prepregs) after storage, or elseextrusion, pultrusion, winding, and resin transfer molding (RTM) calledresin infusion technologies (RI).

The fibers can be impregnated with the composition of the invention, inparticular, and thereafter cured at a higher temperature. In the courseof impregnation and any subsequent storage period, curing does not beginor is only minimal.

As casting compounds for embedding, attaching or solidifying shapedarticles, the compositions are employed, for example, in electronicsapplications. They are suitable as flip-chip underfill or as electricalcasting resins for potting, casting and (glob-top) encapsulation.

EXAMPLES Starting Materials

The epoxy compound used was the diglycidyl ether of bisphenol A (DGEBAfor short), available as a commercial product from Nan Ya under the nameNPEL 127H.

Compositions Investigated

In each case 5 parts by weight of the imidazolium salt or of a mixtureof imidazolium salts were mixed with 100 parts by weight of the epoxycompound. Table 1 lists the compositions and results. In the cases of1^(×) and 1^(××), mixtures with further ingredients as well were tested(see footnote below Table 2).

Measurement Methods

The onset and the process of curing were investigated by means ofDifferential Scanning Calorimetry (DSC). For this purpose, 5 to 15milligrams of the composition were heated in a DSC calorimeter (DSC 822,Mettler Toledo) with a constant rate of 10° C./min.

The parameters determined were To (beginning of exothermicpolymerization reaction, onset temperature, Tmax (temperature maximum ofthe exothermic peak, corresponding to the maximum acceleration ofreaction), and ΔH (integral of the DSC curve, corresponding to the totalamount of liberated heat from the polymerization reaction).

Measured in addition were the glass transition temperature (Tg) of thecured, fully reacted sample, by means of DSC as follows:

20 g of the uncured composition were introduced with a film thickness of3 to 4 mm into an aluminum boat and cured for 30 minutes each at 40° C.,60° C., 80° C., 100° C., 120° C., and 140° C. The Tg of the cured samplewas determined by DSC measurement with a heating rate of 30° C./min, asthe average value from three independent measurements.

The storage stability (pot life) was examined by measuring the relativeviscosity (GELNORM®-RVN viscometer). At different temperatures (25° C.,80° C., 100° C., and 120° C.) a measurement was made of the time, indays (d) or minutes (min). The time reported is the time after which themixture is still pourable.

TABLE 1 Imidazolium salts Salt Imidazolium cation Anion 11-Ethyl-3-methylimidazolium Thiocyanate 3 1,3-DimethylimidazoliumAcetate C1 1-Butyl-3-methylimidazolium Chloride C21-Ethyl-3-methylimidazolium Methanesulfonate

TABLE 2 Results of the DSC measurement and storage stability StorageStorage stability stability To Tmax ΔH Tg 25° C. 80° C. Salt (° C.) (°C.) (J/g) (° C.) (d) (min) 1 93 123 533 127 2 17 C1 192 245 518104 >20 >300 C2 280 315 418 n.d. n.d. >300

1. A method of curing a composition, comprising curing a compositioncomprising at least one epoxy compound in the presence of a latentcatalyst comprising a 1,3-substituted imidazolium salt of formula I

wherein R1 and R3 are each an organic radical having 1 to 20 C atoms,R2, R4, and R5 are each an H atom or an organic radical having 1 to 20 Catoms, it also being possible for R4 and R5 together to form analiphatic or aromatic ring, and X is thiocyanate anion.
 2. The methodaccording to claim 1, wherein R1 and R3 are each a C1 to C10 alkylgroup, an allyl group or a benzyl group.
 3. The method according toclaim 1, wherein R2, R4, and R5 are each an H atom or a C1 to C8 alkylgroup, a C1 to C8 alkenyl group, more particularly an allyl group, aphenyl group or a benzyl group.
 4. The method according to claim 1,wherein R2 is an H atom.
 5. The method according to claim 1, wherein thecurable composition comprises an epoxy compound having at least 2 epoxygroups.
 6. The method according to claim 1, wherein the curablecomposition comprises an epoxy compound having on average 2 epoxygroups.
 7. The method according to claim 1, wherein the compositioncomprises an epoxy compound obtainable by reacting epichlorohydrin withalcohols.
 8. The method according to claim 1, wherein the composition iscomposed of at least 30% by weight, of epoxy compound, in addition towater and organic solvent.
 9. The method according to claim 1, whereinthe composition further comprises at least one additional ingredient.10. The method according to claim 1, wherein the amount of the latentcatalyst is 0.01 to 10 parts by weight per 100 parts by weight of epoxycompound.
 11. A curable composition comprising an epoxy compound and alatent catalyst of formula I.
 12. The curable composition according toclaim 11, composed of at least 30% by weight of an epoxy compound, inaddition to water and organic solvents.
 13. A method of curing acomposition according to claim 11, wherein the curing takes place attemperatures less than 200° C.
 14. (canceled)
 15. The method accordingto claim 9, wherein the additional ingredient is an anhydride curingagent, a phenolic resin, or a novolak.
 16. The method according to claim1, wherein the latent catalyst is represented by formula I.
 17. Themethod according to claim 1, wherein the curing comprises a polyadditionreaction.